Stepping motor with rotor and stator having related axial width

ABSTRACT

A stepping motor capable of improving the degree of smoothness of the rotation of the rotor thereof is provided. According to the present invention, when the width of each of magnets (41, 42, or 44, 45) in the axial direction of the rotor (3) of the stepping motor is expressed by W0, the width of overlapping of each of sets of pole teeth 15, 16 and 25, 26 in the axial direction of the rotor 3 is expressed by W1 and the width of a space between the inner surfaces of each of outer yokes(14, 24) and each of inner yokes(13, 23) is expressed by W2, the width W0 of each of the magnets (41, 42) is so set as to satisfy the relationship of W2&gt;W0&gt;W1.

TECHNICAL FIELD

The present invention relates to a permanent magnetic type steppingmotor and more particularly to a stepping motor useful as a movement ofan analogue type indicating meter.

BACKGROUND TECHNOLOGY

Conventionally, there has been a permanent magnet type stepping motor asa structure widely used as an actuator part in such fields as OAequipment, home electrical appliances, automobiles and the like and inmany cases, it is provided with an annular stator constructed bycombining an exciting coil and a yoke, and a rotor comprising apermanent magnet axially and rotatably supported within a hollow portionof the annular stator. To the peripheral surface of the rotor opposingto the annular stator, a plurality of magnetic poles N and S aremagnetized at a constant pitch in an alternative fashion. Further, inthe annular stator, a plurality of pole teeth extending to the hollowportion of the stator from the yoke and opposing to the magnetic pole ofthe rotor are formed at a constant pitch in an annular comb-shapearrangement so that when a pulse signal is supplied to the excitingcoil, the yoke including the pole teeth is magnetized and the rotorrotates every predetermined rotational angle by a magnetic actionbetween the pole teeth and the magnetic poles of the rotor. Since thisstructure has various kinds of advantages such as the capability ofachieving a high driving torque in spite of its compactness, attentionhas been directed to this type of stepping motor of late as a movementof an analogue type indicating meter such as a speedometer and an enginetachometer, for example.

In the case of using this kind of stepping motor as a movement of theanalogue type indicating meter, the rotor and the indicator may beconnected through a rotary shaft and the rotor interlocked with theindicator may be driven by supplying a pulse signal in response to ameasured amount but since the stepping motor operates the rotor stepwisein correspondence to the pitch of the pole teeth, the indication of themeasured amount by means of the indicator connected to the rotor alsonecessarily becomes stepwise, that is, intermittent, so that therotation of the motor becomes irregular. Accordingly, in the case ofusing the stepping motor as a movement of the meter, there are proposed,as disclosed in Japanese Unexamined Patent Publication No. 6-38497, astructure in which in order to prevent the rotary shaft of the rotor andthe indicator from being directly connected, a reduction gear mechanismcomprising a plurality of gears is disposed between them, or as shown inJapanese Unexamined Patent Publication No. 6-82271, a structure in whicha pulse signal(voltage) having a stepped waveform closely similar to asine wave is applied to each of the exciting coils, thereby finelydriving the motor stepwise, a so called micro-step drive.

However, in the former structure using the reduction gear mechanism, inaddition to the motor itself becoming large-sized, the number of partsis increased to make it difficult to assemble them with ease, so that itis disadvantageous in view of its productivity, and further, there is afear that the problem of backlash and the like due to the employment ofthe reduction gear mechanism may arise. On the contrary, in the case ofthe latter structure using the micro-step drive, it is possible tosoften the intermittent motion of the rotor (the indicator) to a level,in which no visual irregularity of the rotor is observed, by finelydriving the rotor stepwise according to a predetermined divisionalnumber. However, the rotational angular velocity of the rotor at everystep tends to scatter due to an unbalance of the magnetic circuit formedbetween the magnetic pole of the rotor and the yoke including the poleteeth at every step of the rotor so that it is not sufficient in view ofthe smoothness of rotation of the rotor.

The present invention has been made by taking these points intoconsideration and a main object of the invention is to improve thesmoothness of rotation of a rotor itself by designing a rotor-yokestructure, and in the case of using the structure as a movement for ananalogue type indicating meter, to provide a stepping motor which cansecure the rotation of an indicator with no physical disorder withoutthe necessity of providing a reduction gear mechanism.

SUMMARY OF THE INVENTION

The stepping motor of the present invention is constructed such that afirst annular stator comprising a pair of inner and outer yokes forminga magnetic path for a first exciting coil and a second annular statorcomprising a pair of inner and outer yokes forming a magnetic path for asecond exciting coil are concentrically laminated in such a manner thatthe respective inner yokes come to lie adjacent to each other so as toform a stator-laminated body; a rotor is rotatably supported by a rotaryshaft within a hollow portion of the stator-laminated body, the rotorbeing formed of first and second magnets each having its outerperipheral surface alternately and heteropolarly magnetized with aplurality of magnetic poles along the rotational direction thereof andconcentrically fixed to the rotary shaft or being formed of a plasticmagnet having its outer peripheral surface alternately and heteropolarlymagnetized with a plurality of magnetic poles along the rotationaldirection thereof and having a small-diameter portion at the center, andlarge-diameter portions at both ends, thereof, so as to correspond tothe annular stators, respectively; and two sets of pole teeth in acomb-teeth type arrangement extending into the hollow portion of thestator-laminated body from the inner and outer yokes along the axis ofthe rotary shaft of the rotor so as to face the first and second magnetsor the first and second large-diameter portions of the plastic magneticrotor, wherein the width of each of the first and second magnets or thewidth of each of the large-diameter portions of the plastic magnet ismade larger than the width of overlapping of the pole teeth making eachof the pole teeth sets and smaller than the width of the space betweenthe inner surfaces of each of the sets of inner and outer yokes makingpairs by sandwiching each of the sets of pole teeth therebetween, sothat the degree of smoothness of rotation of the rotor is improved andespecially when the stepping motor is used as a movement of an analogueindicating meter, it is possible to obtain the:rotation of the motorfree of any physical disorder without the necessity of using a reductiongear mechanism.

Further, in this case, when the width of each of the first and secondmagnets or each of the first and second large-diameter portions is soset as to lie outside the center of the space between the inner surfacesof the inner and outer yokes, the degree of smoothness of rotation ofthe rotor can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the external appearance of a steppingmotor according to a first embodiment of the present invention;

FIG. 2 is an exploded cross sectional view of the stepping motor shownin FIG. 1;

FIG. 3 is a cross sectional view of the stepping motor shown in FIG. 1;

FIG. 4 is an illustrative view showing a developed state of a statortogether with a rotor;

FIG. 5 is a waveform diagram showing the waveform of a pulse signalsupplied to an exciting coil;

FIG. 6 is a graph showing the rotational characteristic of the rotoraccording to the first embodiment of the invention; and

FIG. 7 is a cross sectional view showing a rotor according to a secondembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of the external appearance of a steppingmotor according to a first embodiment of the present invention, FIG. 2is an exploded cross-sectional view of the stepping motor of FIG. 1 withthe view illustrating the internal structure of the stepping motor andFIG. 3 is a cross sectional view of the stepping motor of FIG. 1 withthe view illustrating how the stepping motor is assembled. Particularlyin FIGS. 1 and 2, the stepping motor according to the instant embodimentmainly comprises first and second annular stators 1 and 2, a rotor 4having a rotary shaft 3 at the center thereof and first and secondbearing plates 5 and 6 which axially support the rotor 4 through therotary shaft 3.

In FIG. 2, the first annular stator 1 comprises a hollow bobbin 12having a first annularly wound exciting coil 11, a cup-shaped outer yoke13 in the form of a torus having a central through hole and a planarinner yoke 14 having a central through hole and disposed in the openingof the outer yoke 13 wherein the first annular bobbin 12 is disposedbetween the outer and inner yoke 13 and 14 in a housed state, aplurality of pole teeth 15 and 16 are formed at the peripheral edges ofthe through holes of the outer and inner yokes 13 and 14, respectively,along the direction of axis of the rotary shaft 3 of the rotor 4 at aconstant pitch and these pole teeth 15 and 16 are alternately combinedwith each other within the hollow portion of the first annular bobbin 12so as to face the peripheral surface of the rotor 4 in an annular combtype arrangement.

The second annular stator 2 has also the same structure as the firstannular stator 1 and comprises a second hollow annular bobbin 22 havinga second annularly wound exciting coil 21, a cup-shaped outer yoke 23 inthe form of a torus having a central through hole and a planar inneryoke 24 having a central through hole and disposed in the opening of theouter yoke wherein the second annular bobbin 22 is disposed between theouter and inner yokes 23 and 24 in a housed state, a plurality of poleteeth 25 and 26 are formed at the peripheral edges of the through holesof the outer and inner yokes 23 and 24, respectively, along thedirection of axis of the rotary shaft 3 of the rotor 4 at a constantpitch, and these pole teeth 25 and 26 are alternately combined with eachother within the hollow portion of the second annular bobbin 22 so as toface the peripheral surface of the rotor 4 in an annular comb typearrangement.

Further, the first and second annular stators 1 and 2 are concentricallylaminated to each other along the direction of axis of the rotary shaft3 of the rotor 4 in such a manner that the yokes 14 and 24 lie adjacentto each other, and as shown in FIG. 3, constitute a laminated body ofthe annular stators 1 and 2 with a hollow portion S formed at the centerthereof.

In the instant embodiment, the rotor 4 comprises first and secondcylindrical magnets 41 and 42 made of, for example, ferrite, and fixedto the rotary shaft 3 through a magnet holder in concentric andspaced-apart relationships with each other, and a plurality of magneticpoles N and S extending to the direction of axis of the rotary shaft 3are alternately magnetized to the peripheral surfaces of the first andsecond magnets 41 and 42 along the rotational direction at a constantpitch, so that the phases of the magnetic poles of the magnets 41 and 42are made to coincide with each other.

Then, the rotor 4 is received within the hollow portion S of thelaminated body of the annular stators 1 and 2 and rotatably supported bythe first and second bearing plates 5 and 6 fixed to the annular stators1 and 2, respectively, so that the magnetic poles of the magnets 41 and42 are respectively opposed to the pole teeth 15, 16 and 25, 26 of theinner and outeryokes 14, 13 and 21, 23 at a predeterminedinterval.(refer to FIG. 3)

FIG. 4 is an illustrative view showing a developed state of the stators1 and 2 together with the rotor 4 so as to illustrate a detailedpositional relationship between each of the annular stators 1 and 2 andthe rotor 4 according to the first embodiment wherein the pole teeth 15,16 and 25, 26 are each formed to have an equal basic pitch P, the phaseof the arrangement of the pole teeth 15, 16 making a pair and that ofthe pole teeth 25, 26 making a pair are shifted from each other by 1/2 Pin the rotational direction of the rotor 4 and accordingly, the poleteeth 15 and the pole teeth 26 which are most separated from earth otherare shifted from each other by 1/4 P in the rotational direction of therotor 4.

In this case, the relationship between the stators 1, 2 and the rotor 4is so established that when the width of each of the magnets 41 and 42in the rotational direction of the rotary shaft 3 is expressed by W0,the width of overlapping of the pole teeth 15 and 16 and the width ofoverlapping of the pole teeth 25 and 26 are each expressed by W1 and thespace between the internal surfaces of the outer and inner yokes 13 and14 sandwiching the pair of teeth 15, 16 therebetween and the spacebetween the internal surfaces of the outer and inner yokes 23 and 24sandwiching the pair of teeth 25, 26 therebetween (that is, thethickness' of the first and second bobbins in the axial direction) areeach expressed by W2, the relationship of W2>W0>W1 is satisfied.

The stepping motor having the above-described structure according to theinstant embodiment is driven in such a manner that a pulsating signal(voltage) having a waveform shown in FIG. 5, for example, is supplied tothe first and second exciting coils 11 and 21 from a drive circuit (notshown). This pulsating signal is a signal having a SIN or COS waveformwhich becomes a typical approximate waveform for effecting a mechanicalstepped motion (a mechanical rotational angle) determined by the pitch Pof the pole teeth 15, 16 and 25, 26 of the outer and inner yokes 12, 13and 23, 24 defined by a so called micro step drive (every rotationalangle which is obtained by dividing the mechanical rotational angle intoa predetermined number of divisions) and by such microstep drive, therotor 4 can be rotated clockwise and counter-clockwise by everyrotational angle in correspondence to the divided number. Thus, when anindicator (not shown) is fixed to the rotary shaft 3 and the drivecircuit is caused to perform a microstep drive in correspondence to themeasured value of the velocity and the like of a vehicle, the rotor canbe used as a movement for an analogue type measuring instrument.

Now, the reason why the above-mentioned width W0 is so set as to satisfythe relationship of W2>W0>W1 will be explained. This size setting is aresult of experiments in which in order to achieve the smoothness ofrotation of the rotor 4, a plurality of kinds of rotors having differentW0 sizes and a plurality of sets of outer and inner yokes 13, 14 and 23,24 having different W1 and W2 sizes, respectively, were prepared andcombinations of these sets of rotors and yokes were compared with oneanother. According to these experiments, it has been found that as shownin FIG. 6, the rotational characteristic of the rotor 4 shown by a solidline has a degree of smoothness of not more than 10% when the width W0is larger than the width W1 and smaller than the width W2. That is, inFIG. 6, the degree of smoothness (%) shown along a vertical axis is avalue which was obtained in such a manner that an optical rotary encoderwas fixed to the rotary shaft 3 so as to measure the (rotational)angular velocity (deg/sec) of the indicator, an average value A of theangular velocity per rotation of the indicator (the rotor 4) and thedistribution (the reference value B of variation) of the angularvelocity during one rotation of the indicator were obtained and both ofthe values were calculated on the basis of a calculation formula ofB/A×100. From this value, it will be seen that the smaller the value(%), the greater the degree of smoothness of rotation. Further, thevalue of the degree of smoothness of not more than 10% is a criterion(the minimum value) at which no physical disorder takes place withrespect to the rotation of the indicator when, for example, theindicator is mounted to the rotary shaft 3 and the stepping motor isused as a movement of the analogue type indicating meter.

Further, in the instant embodiment, by shifting the center C1 of thewidth W0 outwardly of the center C2 of the width W2 (W1) as shown inFIG. 4, a smoother rotational characteristic of the rotor 4 as shown bya dotted line in FIG. 6 was obtained as a result of comparison andverification by experiments of each of the cases where the center C1 isinside the center C2 and the center C1 is outside the center C2 as shownin FIG. 4.

Thus, in the instant embodiment, the first annular stator 1 comprising aset of inner and outer yokes 14 and 13 forming the magnetic path of thefirst exciting coil 11 and the second annular stator 2 comprising a setof inner and outer yokes 23 and 24 forming the magnetic path of thesecond exciting coil are concentrically laminated in such a manner thatthe inner yokes 14 and 24 come to lie adjacent to each other so as toform the laminated body of the stators 1 and 2, the rotor 4 is rotatablysupported within the hollow portion S of the laminated body of thestators 1 and 2 through the rotary shaft 3, the rotor 4 is constitutedby the first and second magnets 41 and 42 each having a peripheralsurface to which a plurality of magnetic poles are alternately andheteropolarly magnetized along the rotational direction of the rotor 4and concentrically fixed to the rotary shaft 3 and two sets of poleteeth extending into the hollow portion S along the axis of the rotaryshaft 3 of the rotor 4 from the inner and outer yokes 14, 24 and 13, 23and opposing to the first and second magnet 41 and 42 of the rotor 4 inthe annular comb type arrangement, and wherein the width W0 of each ofthe first and second magnets 41 and 42 is set larger than theoverlapping width W1 in the axial direction of the rotary shaft 3 of therespective pole teeth forming the respective pair in correspondence tothe magnets 41 and 42, and smaller than the width W2 between the innersurfaces of the inner and outer yokes 14, 13 and 24, 23 forming the pairwith respect to the respective pair of the pole teeth, so that thesmoothness of rotation of the rotor 4 can be improved, and in the caseof using it as a movement for an analogue type indicating meter, therotation of an indicator free of physical disorder can be obtainedwithout the necessity of using a reduction gear mechanism.

Further, when the width W0 is set to lie outside the center of the widthW2 between the inner surfaces of the inner and outer yokes 14, 13 and24, 23, the degree of smoothness of rotation of the rotor can be furtherimproved.

FIG. 7 is a cross sectional view of the rotor according to a secondembodiment of the present invention wherein the rotor 4 is constructedby a plastic magnet 47 whose outer peripheral surface is alternately andheteropolarly magnetized with a plurality of magnetic poles along therotational direction thereof and which has a small-diameter portion 44at the center and first and second large-diameter portions 45 and 46 atboth ends thereof, respectively, so as to correspond to the annularstators 1 and 2. In this case, the dimensional relationship of W2>W0>W1with respect to the width W0 of the rotor is established and it ispossible with such rotor structure to obtain the same effect as in thecase of the above-described first embodiment.

Industrial Availability

As described above, the present invention has been explained withreference to a two-phase or four-phase permanent magnet type steppingmotor but the invention is not always limited thereto and can also beapplied to a single- or multi-face (more than four-phase) permanentmagnet type stepping motor structure.

What is claimed is:
 1. A stepping motor comprising:a first annularstator comprising an inner yoke and an outer yoke for forming a magneticpath for a first annularly wound exciting coil, a second annular statorcomprising an inner yoke and an outer yoke for forming a magnetic pathfor a second annularly wound exciting coil and laminated concentricallywith the first annular stator, and a rotor rotatably supported by arotary shaft within a hollow portion of a stator-laminated body formedby concentrically and vertically laminating the first and second annularstators with the inner yokes thereof lying adjacent to each other,wherein said rotor comprises a first and a second magnet each having aplurality of magnetic poles heteropolarly magnetized to the outerperipheral surface thereof in an alternative fashion along therotational direction thereof and concentrically fixed to said rotaryshaft, wherein two sets of pole teeth are disposed on said first andsecond annular stators, respectively, in an annular comb-teetharrangement such that they extend into the hollow portion of saidstator-laminated body from said inner and outer yokes along the axis ofsaid rotor, wherein when the axial width of each of the first and secondmagnets is expressed by W0, the axial width of overlapping of each ofthe two sets of pole teeth corresponding to said first and secondmagnets is expressed by W1 and the width of a space between the innersurfaces of each of said inner and outer yokes making a pair sandwichingeach of said sets of pole teeth therebetween is expressed by W2,W2>W0>W1; and wherein the center of said width W0 is offset towards saidouter yokes with respect to the center of said width W2.
 2. A steppingmotor comprising:a first annular stator comprising an inner yoke and anouter yoke for forming a magnetic path for a first annularly woundexciting coil, a second annular stator comprising an inner yoke and anouter yoke for forming a magnetic path for a second annularly woundexciting coil and laminated concentrically with the first annularstator, and a rotor rotatably supported by a rotary shaft within ahollow portion of a stator-laminated body formed by concentricallylaminating the first and second annular stators with the inner yokesthereof lying adjacent to each other through the rotary shaft, whereinsaid rotor comprises a magnet whose peripheral surface is heteropolarlymagnetized with a plurality of magnetic poles in an alternative fashionalong the rotational direction thereof and which has first and secondlarge-diameter sections at both ends thereof so as to correspond to saidfirst and second annular stators, respectively, wherein two sets of poleteeth are disposed on said first and second annular stators,respectively, in a comb teeth arrangement so as to face said first andsecond large-diameter portions such that they extend into the hollowportion of said stator laminated body from said inner and outer yokesalong the axis of said rotor, wherein when the axial width of each ofsaid first and said second large-diameter section is expressed by W0,the axial width of overlapping of each of said two sets of pole teethcorresponding to said large-diameter sections is expressed by W1 and thewidth of a space between the inner surfaces of each of said inner yokesand each of said outer yokes is expressed by W2, W2>W0>W1; and whereinthe center of said width W0 is offset towards said outer yokes withrespect to the center of said width W2.